[Effect of radiation on the formation of isolabels in the late replicating regions of the Chinese hamster genome].

The cells synchronized in the G1 stage were incubated in a medium containing 5-bromodeoxyuridine, and chromosomes were analysed after two rounds of replication Iso-labels (IL) were observed in the late-replicating (heterochromatic) segments. The frequency of IL increased after irradiation during the G2 stage of the second cycle. Moreover, IL were observed in euchromatic segments (not recorded in control cells), whereas the frequency of sister chromatid exchanges decreased. Formation of IL in heterochromatin follows the statistical regularities: IL were only observed in some fraction of cells and, as a rule, involved not all heterochromatic segments. The regularities established witness against the uninemic model of chromosome.